The operations of many floating vessels (e.g. semi-submersible drilling rigs, drill ships and pipe-laying ships) are impeded by sea swell. Sea waves impart an up and down motion to the vessel (known as ‘heave’), the period of which can range from a few seconds to 25 s or so, and can be of a few centimeters to 15 m or more in amplitude. This up and down motion is imparted to a load attached to the vessel. In many circumstances the motion of the load is highly undesirable and even dangerous to equipment and personnel. For example when attempting to drill a wellbore in the sea bed, the motion can cause a corresponding motion of the drill string. The up and down movement of the drill bit is highly undesirable and severely restricts the operating window of the rig. For example, it is estimated that in the North Sea as much as 20% of rig operating time is lost ‘waiting on weather’ i.e. waiting for better weather when the sea is calmer.
Heave compensation is concerned with reducing the effect of this up and down motion on a load attached to the vessel. So-called ‘passive’ heave compensation methods are known which rely on the load being fixed at some other point (e.g. to the sea bed). Sea swell causes the vessel to move relative to the load and a passive compensator uses compressed air to provide a low frequency damping effect between the load and the vessel. There are several disadvantages with passive heave compensation methods and apparatus, including that the weight (typically 100-150 tons) of the passive compensator must be suspended tens of meters above the rig floor, which affects the centre of gravity of the vessel, and that the use of passive compensation is limited to loads that are attached to some other point.
So-called active heave compensation methods have been deployed in the field in recent years. An active heave compensation method involves measuring the movement of the vessel using a measuring device (for example a Motion Reference Unit or MRU) and using a signal representing the motion of the vessel to control a drive for moving the connection device (e.g. travelling block, crane hook) relative to the vessel. In principle, if the connection device is moved in a manner equal but opposite to the motion of the vessel the heave can be substantially cancelled. A major advantage of active heave compensation is that it does not rely on movement of the load itself relative to the vessel before compensation can be applied.
The drive controlling motion of the connection device may comprise an AC drawworks for example. A drawworks is a powerful (e.g. 6 MW) winch that is connected to the connection device by a cable that passes through a block and tackle arrangement. Reeling in and out of the cable causes the connection device to be raised and lowered relative to the vessel. An operator command (e.g. to raise or lower the load) is superimposed onto the heave compensation so that the desired motion of the load is achieved irrespective of the motion of the vessel.
However, it has been observed in the field that such active heave compensation has an error of about 10-20% of the heave amplitude. Thus for a heave amplitude of 1 m, the load can be out of position by about 0.1-0.2 m. Such an error is not acceptable, particularly if lowering a string of tubulars from the surface for connection to wellhead on the sea bed for example.
Another problem of active heave compensation is that as the amplitude of the motion of the vessel due to heave increases, the demands on the drive to achieve complete compensation increases accordingly. Sooner or later each parameter of the drive will reach its limitation: force limitations, power limitations, speed limitations, and/or acceleration limitations. In this situation it can be dangerous to keep the active heave compensation in operation since some part may fail and damage equipment and/or personnel; yet at the same time it is dangerous to switch off active heave compensation since the heave of the vessel may cause similar problems. The usual method for dealing with this problem has been to set a constant threshold for each of the parameters; if one of parameters exceeds the constant threshold active heave compensation is switched off. However, this does not really address the aforementioned problem.
A yet further problem of active heave compensation is caused in activating and de-activating the compensation. In particular, large torque fluctuations can result in the motors controlling the load when heave compensation is switched on or off.